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WHT azimuth oscillations

Over the years, oscillations in azimuth and elevation have occasionally been seen at the WHT, but these have been rare and short-lived, and have had little impact on observing. In December 2013 oscillations in azimuth suddenly began to occur more frequently. By early 2014, ~ 3% of the observing time at the WHT was being lost due to oscillations. This was a source of considerable frustration for affected observers.

Tracing the origin of the oscillations, which could not be reproduced during the day, proved surprisingly difficult, and required a thorough review of the mechanical, electronics and software components involved.

A breakthrough came on 25 February 2014 when ING electronics engineers identified a failing capacitor in the azimuth power amplifier:

After replacing the degraded capacitor, the azimuth oscillations have not been seen again.

ING continues to monitor closely the tracking performance of the WHT, but we are reasonably confident that the oscillations problem has been solved.

We would like to thank observers for their patience when faced with the loss of telescope time.

For their role in resolving this stubborn problem, we congratulate the staff of ING's telescope and instrument group, the staff of the ING operations team, and Martin Fisher (ex-ING), who helped our teams to analyse the diagnostic data.

(The above text is taken from Marc Balcells' email to the INGNEWS mailing list on 11 March 2014.)

Technical backgound:
This is the 68-μFarad tantalum capacitor which caused the trouble:

It has now been replaced by 3 smaller capacitors mounted in parallel, visible top-right (red) on this image of a small part of one of the power-amplifier circuit boards:

The failing capacitor was found mounted the wrong way round, and after 30 years of use, this had probably contributed to the degradation of the component. There were no external signs of damage. Inspection of similar power-amplifier modules at ING showed that the same component was mounted the wrong way round in 4 out of 5 cases.

The function of the capacitor and the accompanying resistor is to delay by a few seconds, after power-on, the de-activation of a relay preventing current from reaching the motors (i.e. while demand is stabilising).

Interestingly, these are almost the only components on the circuit board whose location is not marked up in yellow (the 'silkscreen'), suggesting that they were a late addition to the design.

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Last updated: 20 March 2014

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Contact:  (WHT Manager)
Last modified: 20 March 2014